<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8"> 
<title>菜鸟教程(runoob.com)</title> 
<style>
.Site {
  display: flex;
  min-height: 100vh;
  flex-direction: column;
  border: 1px solid red;
}

.Site-content {
  flex: 1;
  border: 1px solid red;
}

</style>

</head>

<body class="Site">
  <header>…</header>
  <main class="Site-content">…</main>
  <footer>…</footer>
</body>

</html>